Turbocharger arrangements are known in many different configurations and are an important part of many drives, especially in the motor vehicle sector. To increase the performance of such turbocharger arrangements, concepts involving a plurality of compressor wheels driven via a shaft by a common turbine are known. Series or parallel flow circuits comprising such compressor stages, depending on the pressure or mass flow requirements, are known.
Particularly parallel circuits comprising compressor stages that discharge air into a common volume tend to be more difficult to control in terms of flow dynamics than turbochargers having just one compressor wheel. This is due to the fact that the two compressor stages generally have different flow properties, and, particularly close to or beyond the surge limit, this leads to the flow conditions first of all becoming unstable at one compressor wheel, something that can lead to an effect on the flow conditions at the other compressor wheel owing to backflows, with the result that this other compressor temporarily operates at a different and possibly likewise unstable operating point. The outcome is that difficult-to-predict oscillating effects arise, these being known as “compressor oscillation” and being incompatible with correct operation of the turbocharger arrangement.
Such compressor oscillation occurs even if there is an attempt to make both compressor stages as similar and symmetrical as possible. The inventors of the present disclosure have recognized the problem underlying this phenomenon to be that, admittedly, the compressor wheel geometry can be made identical and that the outlet duct can also be designed to be symmetrical and substantially similar for both compressor wheels in terms of gas dynamics. However, uniform distribution of the inflowing gas stream between the two compressor wheels is hardly possible in practice since geometrical constraints and installation space considerations lead to different inlet geometries.
Thus, in one possible design of a turbocharger having a multiple compressors, it is envisaged that the common inlet line extends with a substantially circular cross section into a first inlet duct, which branches off in the core region of the cross section and leads over a relatively direct path to the inlet of a compressor wheel, and that a circumferential surface region, in the form of a circular ring, of the cross section of the common inlet duct is routed via a second inlet duct to a second compressor wheel, which lies opposite the first compressor wheel. This second inlet duct is longer and has a plurality of deviations, with the result that this inlet duct has a higher flow resistance. Parallel compressor turbocharger configurations with a bifurcation of the inlet duct are furthermore known. In this case, the supply generally takes place in the axial direction to one compressor wheel and in the radial direction to the other compressor wheel for reasons of installation space. Even if an attempt can be made, by means of the distribution ratio at the branch point of the inlet ducts, to guide equal mass flows into both inlet ducts, this is ultimately hardly possible since the actual distribution ratio is dependent on the actual gas mass flow and on other parameters and therefore varies over the operating range.
Owing to these differing inlet ratios, different inlet pressures are obtained at both compressor wheels. However, since the wheel geometry, i.e. compression properties and outlet pressure—both wheels discharge into the same outlet duct—is/are identical, different operating points (pressure ratios pin/pout) in the characteristic curves and hence different stability behavior of the individual compressor wheels may be obtained.
However, the inventors herein have recognized potential issues with such systems. As one example, to avoid the compressor oscillation described above, turbochargers operate in operating ranges close to the surge limit to be left with a relatively large safety margin, and this impairs the efficiency of the turbocharger arrangement overall.
In one example, the issues described above may be addressed by a turbocharger arrangement for an internal combustion engine, having at least two compressor wheels, which run on a common drive axle and discharge air in parallel into a common outlet duct, wherein an inlet duct routed separately in at least one section ahead of the turbocharger arrangement is provided for each compressor wheel, for a flow connection between the inlet ducts to be provided close to the compressor wheels.
The present disclosure relates to a turbocharger arrangement for an internal combustion engine, having at least two compressor wheels, which run on a common drive axle and discharge air in parallel into a common outlet duct, wherein an inlet duct routed separately in at least one section ahead of the turbocharger arrangement is provided for each compressor wheel, in accordance with the preamble of patent claim 1. The present disclosure furthermore relates to a method for operating a turbocharger arrangement of this kind in accordance with the preamble of patent claim 9.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.